Inverse gas chromatography is used to study the surface properties of the untreated and chemically treated kaolinite samples. Changes in the enthalpy of adsorption for a variety of probes and in the surface energy of clays are measured and the effect of modification of the natural clay after chemical treatment with Na2CO3 is determined. The surface energy of the natural clay increased by the modification due to an increase in the surface area. It can be concluded that the dispersive component of surface free energy, gamma(d)(s), decreases with temperature in the 200-275 degrees C temperature range for both samples. Temperature coefficients of gamma(d)(s) for untreated and modified kaolinites are -0.1185 and -0.3966 mJ/(m(2)degrees C) with the correlation coefficients (R-2) of 0.8479 and 0.965, respectively. From the retention data for polar probes at infinite dilution, information on the accessibility of surface sites to the probes and on the acid-base character of the surface is obtained. The specific free energy of adsorption, the specific enthalpy of adsorption (Delta H-sp), and the specific entropy of adsorption of polar probes on initial and modified kaolinites are determined. The Delta H-sp values correlated with the donor numbers and modified acceptor numbers of the probes to quantify the acidity (K-A) and basicity (K-D) parameters of clay surfaces. The values of K-A and K-D for initial and modified kaolinites are determined to be 0.1202 and 0.2803; 0.0130 and 0.0408 with the correlation coefficients of -0.9805 and -0.9782, respectively. The unmodified clay sample indicated a more acidic character, while the modified clay sample conferred a largely basic character. Consequently, the predominant surface basicity of the modified kaolinite agrees with expectation, hearing in mind the treatment with Na2CO3, taking into consideration that such a modification contributes to a decrease in the hydrophilicity of the surface and also results in the surface showing only weak Lewis acidity.